FIGURE 1. ALD500R Functional Block Diagram
C
REF
R
INT
C
INT
-
V
+
REF
(11)
V
REF
(12)
C
AZ
R
REF
C
INT
(2)
+
-
C
R
C
= 100KΩ
= 0.1µF
C
BUF
(5)
C
AZ
(4)
REF
B
REF
(8)
REF
(7)
REF
INT
SW
SW
R
R
+
-
SW
IN
Buffer
+
Integrator
-
+
V
IN
(14)
+
-
SW
SW
R
R
+
-
Comp1
-
Level
Shift
Comp2
+
C
(17)
OUT
SW
Az
SW
AZ
+
-
SW
SW
SW
R
R
S
Polarity
Detection
DGND
(18)
AGND
(6)
SW
SW
IN
G
-
V
Analog
IN
(13)
Phase
Decoding
Logic
Switch
Control
Signals
REF
Control
Bias
A
B
V
V
N/C N/C
SS DD
(3) (19) (10) (9)
C
B
C
S
(20)
(15) (16)
Control Logic
I
B
(1)
GENERAL THEORY OF OPERATION
GENERAL DESCRIPTION
The ALD500RAU/ALD500RA/ALD500R are integrating
dual slope analog processors, designed to operate on ±5V
power supplies for building precision analog-to-digital
converters. The ALD500RAU/ALD500RA/ALD500R
feature specifications suitable for 18 bit/17 bit/16 bit
resolution conversion, respectively. Together with three
capacitors, tworesistors, andadigitalcontroller, aprecision
Analog to Digital converter with auto zero can be
implemented. The digital controller can be implemented
by an external microcontroller, under either hardware
(fixed logic) or software control. For ultra high resolution
applications, up to 23 bit conversion can be implemented
with an appropriate digital controller and software.
Dual-Slope Conversion Principles of Operation
The basic principle of dual-slope integrating analog to digital
converter is simple and straightforward. A capacitor, CINT, is
charged with the integrator from a starting voltage, VX, for a
fixed period of time at a rate determined by the value of an
unknown input voltage, which is the subject of measurement.
Then the capacitor is discharged at a fixed rate, based on an
external reference voltage, back to VX where the discharge
time, or deintegration time, is measured precisely. Both the
integration time and deintegration time are measured by a
digital counter controlled by a crystal oscillator. It can be
demonstrated that the unknown input voltage is determined
by the ratio of the deintegration time and integration time, and
isdirectlyproportionaltothemagnitudeoftheexternalreference
voltage.
The ALD500R series of analog processors accept
differential inputs and the external digital controller first
counts the number of pulses at a fixed clock rate that a
capacitor requires to integrate against an unknown analog
input voltage, then counts the number of pulses required
to deintegrate the capacitor against a known internal
reference voltage. This unknown analog voltage can then
be converted by the microcontroller to a digital word, which
is translated into a high resolution number, representing
an accurate reading. This reading, when ratioed against
the reference voltage, yields an accurate, absolute voltage
measurement reading.
The major advantages of a dual-slope converter are:
a. Accuracy is not dependent on absolute values of
integration time tINT and deintegration time tDINT, but is
dependent on their relative ratios. Long-term clock frequency
variations will not affect the accuracy. A standard crystal
controlledclockrunningdigitalcountersisadequatetogenerate
very high accuracies.
b. Accuracy is not dependent on the absolute values of
R
INT and CINT, as long as the component values do not vary
The ALD500R analog processors consist of on-chip digital
control circuitry to accept control inputs, integrating buffer
amplifiers, analog switches, and voltage comparators and
a highly accurate, ultra-stable voltage reference. It
functions in four operating modes, or phases, namely auto
zero, integrate, deintegrate, and integrator zero phases.
At the end of a conversion, the comparator output goes
from high to low when the integrator crosses zero during
deintegration. ALD500R analog processors also provide
direct logic interface to CMOS logic families.
through a conversion cycle, which typically lasts less than 1
second.
c. Offset voltage values of the analog components, such
as VX, are cancelled out and do not affect accuracy.
d. Accuracyofthesystemdependsmainlyontheaccuracy
and the stability of the voltage reference value.
2
Advanced Linear Devices
ALD500RAU/ALD500RA/ALD500R
ALD [ ADVANCED LINEAR DEVICES ]
